\begin{tikzpicture}[>=Stealth,
    every node/.style={fill=white, inner sep=1pt},
]
    \pgfmathsetmacro{\r}{1}
    \pgfmathsetmacro{\R}{1.6}
    \pgfmathsetmacro{\a}{4}

    \draw [thick, pattern={mylines[angle=45, distance={5pt}]}]
        (0, \R) arc [start angle=90, end angle=270, radius=\R]
        -- +(\a, 0)
        arc [start angle=270, end angle=450, radius=\R]
        -- cycle;
    \draw [thick, fill=white]
        (0, \r) arc [start angle=90, end angle=270, radius=\r]
        -- +(\a, 0)
        arc [start angle=270, end angle=450, radius=\r]
        -- cycle;

    \draw [dash dot] (-1.5*\R, 0) -- (\a + 1.5*\R, 0);
    \draw [dash dot] (0, 1.2*\R) -- (0, -1.2*\R);
    \draw [dash dot] (\a, 1.2*\R) -- (\a, -1.2*\R);

    \draw [->] (0, 0) -- (200:\r) node [midway] {$r$};
    \draw [->] (\a, 0) -- +(320:\R) node [pos=0.4] {$R$};
    \draw [<->] (0, -0.3-\R) to [xianduan={above=0.3cm}] node {$a$} (\a, -0.3-\R);
\end{tikzpicture}
